1. Field of the Invention
The invention generally relates to thin glasses, in particular to glasses having a thickness of less than one millimeter. More particularly, the invention relates to the confectioning of thin glass ribbons by wounding them into a roll.
2. Description of Related Art
Thin glasses are used in various fields of technology, for example for displays, windows for optoelectronic components, encapsulations, and electrical insulation layers, to mention a few.
In order to be able to handle thin glasses for further processing it is favorable to wind a thin glass ribbon into a roll. Thus, during further processing the glass may be directly unwound from the roll to be processed. However, a problem therewith is that upon winding into a roll bending stresses are generated in the glass. These bending stresses may lead to breakage of the coiled glass ribbon. Already a single fracture may cause a significant problem, since upon unwinding a processing operation has to be interrupted at the break point of the ribbon.
US 2013/0196163 A1 describes a method for bending glass, in which a glass web is laminated onto a reinforcing layer in a manner so that during bending the neutral plane of the bending line lies in the reinforcing layer and the glass web completely lies in the bending-induced compressive stress zone. This requires reinforcing films of a thickness that is a multiple of the glass thickness, and the adhesive of high strength used for the laminate has to exhibit low creeping behavior and will thus be brittle when cured. However, the high strength may imply problems when the adhesive is difficulty detached or cannot completely removed. Anyhow, the removal of the adhesive is an additional processing step necessary before cutting. Moreover, the winding direction is predefined. With respect to the breaking strength of the coiled glass, creeping in the adhesive joint and stress relaxation in the reinforcing layer have to be taken into consideration. If, due to stress relaxation, the neutral plane is shifted into the glass web, the glass comes under tension stress which may even be increased upon unwinding.
U.S. Pat. No. 8,241,751 B2 describes a glass roll with a low instantaneous fracture probability if a minimum bending radius is obeyed for the bends. However, the document does not consider the aspect of fractures occurring with delay. In particular fractures occurring at the edges of the glass ribbon are neglected. For the dimensioning rules described in this document, glass breakage has to be expected after a very short time.
Often, however, a thin glass is not immediately further processed. Rather, it can be assumed that a thin glass roll will be stored for a certain period of time. Also, the transport to a processing plant will take time and cause additional dynamic loads.
Moreover, even for further processed thin glasses, such as glued thin glass sheets or thin glasses in material composites there is a need to avoid failure of the glass element by glass breakage due to tensile stresses in the material caused by the processing.